Waves of Accelerated Motion in a Glacier Approaching Surge: the Mini-Surges of Variegated Glacier, Alaska, U.S.A.

Abstract

ABSTRACT. Periods of dramatically accelerated motion, in which the flow velocity increases suddenly from about 55 cm/d to a peak of 100-300 cm/d and then decreases gradually over the course of a day, occurred repeatedly during June and July 1978-8 1 in Variegated Glacier (Alaska), a surging-type glacier that surged in 1982-83. These "mini-surges" appear to be related mechanistically to the main surge. The flow-velocity peak propagates down glacier as a wave at a speed of about 0.3 km/h, over a reach of about 6 km in length. It is accompanied by a propagating pressure wave in the basal water system of the glacier, in which, after a preliminary drop, the pressure rises rapidly to a level greater than the ice-overburden pressure at the glacier bed, and then drops gradually over a period of 1-2 d, usually reaching a new low for the summer. The peak velocity is accompanied by a peak of high seismic activity due to widespread fresh crevassing. It is also accompanied by a rapid uplift of the glacier surface, amounting to 6-1 1 cm, which then relaxes over a period of 1-2 d. Maximum uplift rate coincides with the peak in flow velocity; the peak in accumulated uplift lags behind the velocity peak by 2 h. The uplift is mainly due to basal cavitation driven by the high basal water pressure, although the strain wave associated with the mini-surge motion can also contribute. Basal cavitation is probably responsible for the pulse of high turbidity that appears in the terminal out flow stream in association with each mini-surge. In the down-glacier reach, where the mini-surge waves are attenuating, the observed strain wave corresponds to what is expected for the propagating pulse in flow velocity, but in the reach of maximum mini-surge motion the strain wave has a form quite different, possibly related to special features in the mini-surge initiation process from that point up-stream. The flow acceleration in the mini-surges is due to enhanced basal sliding caused by the high basal water pressure and the consequent reduction of bed friction. A preliminary velocity increase shortly before the pressure wave arrives is caused by the forward shove that the main accelerated mass exerts on the ice ahead of it, and the resulting preliminary basal cavitation causes the drop in water pressure shortly before the pressure wave arrives. The mini-surge wave propagation is controlled by the propaga tion of the water-pressure wave in the basal water-conduit system. The propagation characteristics result from a longi tudinal gradient (up-glacier increase) in hydraulic conductivity of the basal water system in response to the up-glacier increase of the basal water pressure in the mini surge wave. The mini-surge waves are initiated in a succession of areas situated generally progressively up-glacier during the course of the summer season. In these areas, presumably, melt water that has accumulated in subglacial *Contribution No.3734,

Abrupt glacier motion and reorganization of basal shear stress following the establishment of a connected drainage system

“…Five other vertical-strain sensors were in operation on day 162, but none shows any response, suggesting that longitudinal extension of the ice occurred only locally. This contrasts with behaviours observed during short-term motion events recorded at Findelengletscher, Switzerland (Iken and Bindschadler, 1986), and``mini-surges'' observed at Variegated Glacier, Alaska, U.S.A. (Kamb and Engelhardt, 1987). During each of these events, a zone of compression and subsequent extension passed down-glacier as a wave.…”

Slow Magnetic Relaxation in Condensed versus Dispersed Dysprosium(III) Mononuclear Complexes

“…Variations in glacier flow on timescales of a few days have received much attention over the last 25 years (Harrison et al , 1986; Kamb and Engelhardt, 1987; Mair et al , 2001; Gudmundsson, 2002; Mair et al ,2002b; Mair et al , 2003, Anderson et al , 2004). These short‐term velocity changes, referred to here as glacier speed‐up events, are important because they demonstrate the coupling between a glacier's hydrology and ice‐motion dynamics and may provide clues to the mechanism of large‐scale flow instabilities such as glacier surging (Harrison et al , 1986; Kamb and Engelhardt, 1987).…”

“…Variations in glacier flow on timescales of a few days have received much attention over the last 25 years (Harrison et al , 1986; Kamb and Engelhardt, 1987; Mair et al , 2001; Gudmundsson, 2002; Mair et al ,2002b; Mair et al , 2003, Anderson et al , 2004). These short‐term velocity changes, referred to here as glacier speed‐up events, are important because they demonstrate the coupling between a glacier's hydrology and ice‐motion dynamics and may provide clues to the mechanism of large‐scale flow instabilities such as glacier surging (Harrison et al , 1986; Kamb and Engelhardt, 1987). Recent speculation as to the cause of seasonal and shorter term variations in surface velocities across the Greenland Ice Sheet (Zwally et al , 2002; Joughin et al , 2008; van der Wal et al , 2008; Shepherd et al , 2009) has refocused attention onto subglacial hydrological evolution and its impact on ice dynamics from alpine glacier speed‐up events, though at present the temporal and spatial extent of such observations is insufficient to determine the wider significance of these processes on the dynamics and mass loss of the Greenland Ice Sheet.…”

“…Through horizontal stress gradients in the overlying ice, glacier motion can be influenced over an area larger than that which experiences direct basal forcing (Harbor et al , 1997; Blatter et al , 1998; Nienow et al , 2005). In addition to increases in glacier velocity (Iken et al , 1983; Kamb and Engelhardt, 1987; Kavanaugh and Clarke, 2001), the high subglacial water pressure often leads to a reorganization of the subglacial drainage system that increases its discharge capacity (Stone and Clarke, 1996; Gordon et al , 1998). The subsequent release of the subglacially backed up water and an increased suspended sediment flux often mark the termination of the speed‐up event (Humphrey et al , 1986; Barret and Collins, 1997; Anderson et al , 1999; Schuler et al , 2002).…”

“…Speed‐up events are most commonly documented by measurements of surface motion in combination with proglacial stream discharge and suspended sediment load (Humphrey et al , 1986; Anderson et al , 1999), subglacial water pressure (Iken and Bindschadler, 1986; Kamb and Engelhardt, 1987) or internal ice deformation (Gudmundsson, 2002; Willis et al , 2003; Mair et al , 2008). Kavanaugh and Clarke (2001) identified distinct subglacial hydraulic and mechanical conditions during a speed‐up event, but did not monitor surface motion.…”